1. Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly, to a technique to controlling an exposure time by using an electronic shutter in an image pickup apparatus.
2. Description of the Related Art
In an amplification-type image pickup apparatus such as that known as a CMOS image sensor, there are various known electronic shutter techniques to control starting and ending of accumulation of charges for all pixels over an entire image plane without using a mechanical method of blocking light.
In a technique associated with an electronic shutter disclosed in Japanese Patent Laid-Open No. 2006-246450, a charge generated in a photoelectric conversion part during an exposure period is transferred to a storage part, and, after the end of the exposure period, the photoelectric conversion part is reset to remove charges remaining in the photoelectric conversion part thereby achieving a function of the electronic shutter.
This technique is characterized in that functions are implemented separately by dedicated parts such that the photoelectric conversion part basically performs only photoelectric conversion and charge accumulation during the exposure period is performed by the charge storage part disposed adjacent to the photoelectric conversion part. Note that the charge storage part is disposed separately from the FD region. Because the saturation value of the number of charges in the photoelectric conversion part is small, transferring of charges from the photoelectric conversion part to the charge storage part can be performed using a low voltage. This device can be easily produced by a simple process based on a general CMOS production process, i.e., the production thereof is easier than CCD or other similar devices.
In the technique disclosed in Japanese Patent Laid-Open No. 2006-246450, the resetting of the photoelectric conversion part is performed after exposure is completed. In this technique, charges generated during the resetting period are not used effectively, and thus an improvement in this respect is sought. Furthermore, Japanese Patent Laid-Open No. 2006-246450 also discloses a technique to suppress dark electrons generated between the photoelectric conversion part and the charge storage part by using a buried channel transistor thereby achieving both surface pinning and electron transport during an exposure operation. However, in the buried channel structure, the height of the potential barrier is low compared with a surface channel transistor, and thus a further consideration is sought in terms of effective use of saturated charges in the charge storage part.
This is true not only for the buried channel structure but also for a structure in which an intermediate-level pulse is supplied to the photoelectric conversion part at a particular time in a period in which charges are accumulated in the photoelectric conversion part thereby transferring the charges to the charge storage part. In this case, a further consideration is desirable in terms of effective use of saturated charges in the photoelectric conversion part.
In view of the considerations described above, the present invention provides an image pickup apparatus that has a charge storage part provided separately from a photoelectric conversion part and an FD region in each pixel and that provides both a high speed shutter operation and an expanded large dynamic range.